JP4657536B2 - Semiconductor laser drive device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser driving device.
[0002]
[Prior art]
For example, a so-called transmissive laser sensor includes a light projecting element and a light receiving element facing each other, and the light receiving element changes depending on the light shielding state of an object existing in the optical path of the laser light emitted from the light projecting element to the light receiving element. The detection of the presence of the object and the measurement of the position and dimensions are performed based on the change in the amount of received light. In order to perform stable detection or the like in such a photoelectric sensor, it is necessary to keep the laser light output from the light projecting element constant. For this purpose, conventionally, a configuration has been adopted in which the laser light output is kept constant by driving and controlling the light projecting element by APC (Automatic Power Control) control.
[0003]
A specific configuration and operation will be described with reference to FIGS. In FIG. 3, reference numeral 1 denotes a pulse signal source that outputs a pulse signal having a high level of 5 [v] and a low level of 0 [v], for example. Reference numeral 2 denotes an integrated element (hereinafter referred to as “minus single power supply type element 2”) in which the laser diode 3 and the photodiode 4 are mounted on the same chip, and their plus terminal sides are commonly connected. The positive terminal is connected to a power supply line connected to the pulse signal source 1. Among these, the laser diode 3 has a cathode side connected to a ground line via an NPN transistor (hereinafter referred to as “transistor 5”) and a resistor 6. On the other hand, the anode of the photodiode 4 is connected to the ground line via the resistor 7, and a current corresponding to the amount of light received by the photodiode 4 flows to the resistor 7. The load voltage of the resistor 7 is detected by the detection circuit 8. Is done. The drive control circuit 9 is connected to the detection circuit 8 and a power supply line between the pulse signal source 1 and the positive terminal of the minus single power supply type element 2, respectively. The detection level Va in the detection circuit 8 and the potential level in the power supply line. Vb is read at any time with a predetermined timing. Then, as shown in FIG. 2, the drive signal S corresponding to the level difference between the detection level Va and the potential level Vb is output. Next, in the laser drive circuit 10, an electric signal L corresponding to the drive signal S level from the drive control circuit 9 is applied to the base of the transistor 5.
[0004]
Here, for example, when the laser light output level when the laser diode 3 is turned on is equal to or lower than a predetermined value (when the third pulse wave is output in FIG. 2), the amount of light received by the photodiode 4 And the detection level Va in the detection circuit 8 decreases. On the other hand, since the potential level Vb in the power supply line rises to 5 [v], which is normal, the difference between the detection level Va and the potential level Vb becomes large. Then, an electric signal L having a level higher than that in the initial state is given to the transistor 5 to increase the amount of current supplied to the laser diode 3, thereby operating the laser light output level. On the other hand, when the laser light output level of the laser diode 3 exceeds a predetermined value (when the fourth pulse wave is output in the figure), the potential level Vb remains at 5 [v]. On the other hand, the detection level Va becomes higher than normal and the difference between the levels becomes smaller, and accordingly, a low level drive signal S is applied to the transistor 5 to reduce the amount of current supplied to the laser diode 3 and thereby the laser. Operates to lower the light output level. As a result, the laser light output of the laser diode 3 can be controlled to be kept at a predetermined value.
[0005]
[Problems to be solved by the invention]
By the way, in recent years, instead of the minus single power supply type element 2, an integrated element (hereinafter referred to as “plus single power supply type element 11”) in which the minus terminal sides of the laser diode 3 and the photodiode 4 are commonly connected has been used. Often done. In the plus single power supply type element 11, the above circuit configuration (FIG. 3) cannot be adopted, and a circuit configuration as shown in FIG. 4 is adopted. Each circuit element has the same configuration as shown in FIG.
[0006]
With this configuration, as with the configuration shown in FIG. 3, the level of the electric signal L applied to the base of the transistor 5 is changed in response to the level fluctuation of the laser light output of the laser diode 3, and the laser diode 3 is changed accordingly. Thus, it should be possible to control the laser light output of the laser diode 3 to be constant.
[0007]
However, in the actual circuit, as shown in FIG. 2, the drive signal S from the drive control circuit 9 appears as a waveform in which the rising and falling portions are gently inclined with respect to the pulse signal having a substantially square waveform. . As a result, the following phenomenon occurs. That is, the pulse signal from the pulse signal source 1 rapidly decreases from the high level to the low level. In synchronism with this, the potential on the collector side of the transistor 5 rapidly decreases to the ground level side, the drive of the laser diode 3 is stopped, and the detection level Va in the detection circuit 8 becomes 0 [v]. At this time, the electric signal L of a predetermined level is still supplied to the base of the transistor 5, and a predetermined current flows into the input side of the drive control circuit 9 through the base and collector of the transistor 5 and the input of the drive control circuit 9. A small potential is generated on the side.
[0008]
Then, the drive control circuit 9 outputs a drive signal S corresponding to a minute level difference between the detection level 0 [v] and the potential level, and the drive control circuit 9 again passes through the base and collector of the transistor 5. Current flows into the input side, and the potential level Vb on the input side of the drive control circuit 9 is further improved. Such a phenomenon is repeated until the pulse signal rises from the low level to the high level next time, and the drive signal S level and the electric signal L level are improved. When the pulse signal rises to a high level in this state, a large current flows through the laser diode 3 and light is emitted with a high laser light output. In this case, there is a problem that stable drive control cannot be performed because it takes time until the laser beam output becomes constant by APC control.
[0009]
In order to solve the above problem, it is conceivable to use a PNP transistor instead of the NPN transistor 5 in the circuit of FIG. With this configuration, even if the electric signal L is given to the base when the pulse signal falls, no current flows to the input side of the drive control circuit 9. However, in an actual circuit, as shown in FIG. 2, the pulse signal from the pulse signal source 1 also has a waveform in which the falling portion is slightly inclined and the signal level becomes unstable. Then, a current flows from the emitter to the base of the transistor, and there is a problem that drive control cannot be performed without stability.
[0010]
In the case of using the negative single power supply type element 2 described above, the laser diode 2 is connected between the collector of the transistor 5 and the input side of the drive control circuit 9, so that the base and collector of the transistor 5 are connected. Thus, the current toward the input side of the drive control circuit 9 is blocked. Accordingly, even in an actual circuit, erroneous control due to the delay of the drive signal S is avoided as a result.
[0011]
The present invention has been made in view of the above circumstances, and an object thereof is to provide a semiconductor laser driving device using a so-called plus single power supply type element capable of performing stable driving control.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, a semiconductor laser driving apparatus according to the present invention includes a semiconductor laser element disposed between a pulse signal source and a ground level and driven based on a pulse signal from the pulse signal source, and a semiconductor Light reception for monitoring, which is contained in the same package as the laser element, and whose anode side is commonly connected to the cathode side of the semiconductor laser element and receives light from the semiconductor laser element and outputs a light reception signal corresponding to the amount of light received An element, a current control element connected between the pulse signal source and the semiconductor laser element, for adjusting an amount of current supplied to the semiconductor laser element, a potential level in a connection line between the pulse signal source and the current control element, and for monitoring A drive signal corresponding to the level of the light receiving signal from the light receiving element is applied to the control terminal of the current control element, and the supply current to the semiconductor laser element is determined. Characterized in place includes a feedback control means for feedback control so that a value, and a semiconductor device which prevents a current flowing from the current control element on the input side of the feedback control means.
[0013]
[Action and effect of the invention]
As described in the section of the conventional explanation, in the actual circuit, when the pulse signal from the pulse signal source falls, the drive signal from the feedback control means falls with a delay. This generates a current that flows from the current control element to the input side of the feedback control means. However, the semiconductor laser driving device according to the present invention includes a semiconductor element that blocks current flowing from the current control element to the input side of the feedback control means. Therefore, no current flows from the current control element to the input side of the feedback control means, and the potential level on the input side of the feedback control means always changes almost in synchronization with the pulse signal level. As a result, a semiconductor using a so-called positive single power supply type element in which the semiconductor laser element and the light receiving element for monitoring are contained in the same package, and the cathode side of the semiconductor laser element and the anode side of the light receiving element for monitoring are commonly connected. In the laser drive device, stable drive control can be performed.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described with reference to FIGS.
For example, a transmissive laser sensor includes a light projecting element and a light receiving element facing each other, and the light receiving element changes depending on the light shielding state of an object existing in the optical path of the laser light emitted from the light projecting element and reaching the light receiving element. Based on the change in the amount of received light, the presence of the object is detected and the position and dimensions are measured. The semiconductor laser driving device 20 according to the present embodiment is for maintaining the laser light output from the light projecting element at a predetermined value in order to perform stable detection or the like in such a photoelectric sensor.
[0015]
First, the circuit configuration of the semiconductor laser driving device 20 will be described. In FIG. 1, reference numeral 21 denotes an input terminal of the power supply line L 1 of the semiconductor laser driving device 20, and a pulse signal source 50 is connected to the input terminal 21. A parallel circuit 22 composed of a zener diode and a capacitor is connected between the power supply line L1 and the ground line in order to absorb a surge voltage generated by the pulse signal from the pulse signal source 50. Further, a light emitting diode 23 driven by the pulse signal is connected and functions as an operation indicator lamp. Next, reference numeral 24 denotes a laser diode 25 (corresponding to the “semiconductor laser element” of the present invention) and a photodiode 26 (corresponding to the “monitoring light receiving element” of the present invention) mounted on the same chip, and It is an integrated element (hereinafter referred to as “plus single power supply type element 24”) in which the negative terminal side is commonly connected, and its negative terminal is connected to the ground line. Among these, the laser diode 25 has its anode side connected to the power supply line L1 via a transistor 29, a diode 30 and the like which will be described later.
[0016]
On the other hand, the cathode side of the photodiode 26 is connected to the input side of the detection circuit 27, and the detection circuit 27 outputs a detection signal of a level corresponding to the amount of light received by the photodiode 26 to the comparator 28 ("feedback control means" of the present invention). To the negative terminal side. The negative terminal side of the comparator 28 is connected to the power supply line L1 connected to the input terminal 21. The comparator 28 is connected to the detection signal level Va supplied from the detection circuit 27 and the pulse signal level from the pulse signal source 50. The potential level Vb of the capacitor 31 that changes accordingly is compared, and the drive signal S corresponding to the level difference is referred to as an NPN transistor (hereinafter referred to as “transistor 29”. This corresponds to the “current control element” of the present invention. ) Given to the base. As a result, a supply current corresponding to the drive signal S level flows to the laser diode 25, and the laser diode 25 is pulse-lit.
[0017]
The diode 30 corresponding to the “semiconductor element” of the present invention has its anode side connected to the positive terminal side of the comparator 28 and its cathode side connected to the collector side of the transistor 29.
[0018]
Next, effects of the semiconductor laser driving device 20 according to the present invention will be described.
First, a case where the circuit configuration is ideal will be described. First, when the pulse signal from the pulse signal source 50 rises from a low level to a high level, the potential level Vb of the capacitor 31 on the plus terminal side gradually rises in the comparator 28 while the detection signal level Va on the minus terminal side is Since 0 [v] remains, the drive signal S corresponding to the difference is given to the base of the transistor 29 and the laser diode 25 starts to emit light. Then, the photodiode 26 receives the laser beam from the laser diode 25, and a detection signal having a level corresponding to the received light level is given to the negative terminal side of the comparator 28. Then, the drive signal S corresponding to the difference from the potential level Vb on the plus terminal side is given, and the laser diode 25 emits light with a predetermined laser light output.
[0019]
Next, when the pulse signal from the pulse signal source 50 falls from the high level to the low level, the potential on the collector side of the transistor 29 is rapidly lowered to the ground level, the supply current does not flow to the laser diode 25, and the light emission is stopped. . Along with this, the amount of light received by the photodiode 26 also becomes zero. Accordingly, the potential level Vb on the plus terminal side of the comparator 28 and the detection signal level Va applied to the minus terminal side are both 0 [v], and the drive signal S is not output from the comparator 28. In such an ideal circuit, similarly to the pulse signal waveform of the pulse signal source 50, the drive signal S output from the comparator 28 also forms a substantially square waveform, and in synchronization therewith. The laser diode 25 is pulsed.
[0020]
Here, for example, when the laser diode 25 deteriorates and the laser light output level at the time of lighting decreases (when the third pulse wave is output in FIG. 2), the received light amount level at the photodiode 26 And the detection level Va in the detection circuit 27 decreases. On the other hand, since the potential level Vb in the power supply line L1 has risen to the initial level, the level difference between the detection level Va and the potential level Vb becomes large. Then, an electric signal L having a higher level from the beginning is given to the transistor 29, and the amount of current supplied to the laser diode 25 is increased, so that the laser light output level is increased. On the contrary, when the laser light output level of the laser diode 25 is improved, the detection level Va becomes higher than the initial level with respect to the potential level Vb as it is at the initial level, and the level difference between them becomes smaller, and the level is lowered accordingly. The drive signal S level is supplied to the transistor 29 and operates to lower the laser light output level by reducing the amount of current supplied to the laser diode 25. Thereby, the laser beam output of the laser diode 25 can be kept constant.
[0021]
As described in the section of the conventional description, in an actual circuit, as shown in FIG. 2, the drive signal S appears to have a gentler waveform at the falling portion than the pulse signal. . As a result, current flows to the positive terminal side of the comparator 28 via the base and collector of the transistor 29, and the potential level Vb of the capacitor 31 is improved, causing the comparator 28 to malfunction. However, in this embodiment, in order to prevent a current flowing from the collector side of the transistor 29 to the positive terminal side of the comparator 28, a diode 30 is connected between them. Therefore, as in the case of the ideal circuit configuration described above, when the pulse signal falls, no current flows into the capacitor 31 on the positive terminal side of the comparator 28, and the potential level Vb of the capacitor 31 is always pulsed. It changes almost synchronously with the signal level. Thereby, stable drive control can be performed in the semiconductor laser drive device 20 using the plus single power supply type element 24. Further, it is possible to avoid a state in which a large current suddenly flows through the semiconductor laser element, and it is possible to suppress deterioration of the semiconductor laser element and the monitor light-receiving element accompanying this.
[0022]
<Other embodiments>
The present invention is not limited to the above-described embodiment. For example, the embodiments described below are also included in the technical scope of the present invention, and various other than the following can be made without departing from the scope of the invention. It can be changed and implemented.
(1) In the above embodiment, the diode 30 is used as the semiconductor element. However, the present invention is not limited to this. For example, a transistor or the like can be used as long as the current flowing from the transistor 29 to the positive terminal side of the comparator 28 can be blocked. There may be.
[0023]
(2) In the above embodiment, the plus single power supply type element 24 in which the laser diode 25 and the photodiode 26 are mounted on the same chip has been described as an example. However, if the ground line is commonly connected, The laser diode and the photodiode may be mounted on separate chips.
[Brief description of the drawings]
FIG. 1 is a circuit configuration diagram of a semiconductor laser driving device according to an embodiment of the present invention. FIG. 2 is a timing chart of a pulse signal, a potential level, a detection level, and a driving signal. Fig. 4 is a circuit configuration diagram of a semiconductor laser driving apparatus using a plus single power supply type element.
DESCRIPTION OF SYMBOLS 20 ... Semiconductor laser drive device 24 ... Plus single power supply type element 25 ... Laser diode 26 ... Photodiode 28 ... Comparator 29 ... Transistor 30 ... Diode 50 ... Pulse signal source L1 ... Power supply line
S ... Drive signal
Va: Detection signal level
Vb: Potential level

Claims (1)

A semiconductor laser element disposed between a pulse signal source and a ground level and driven based on a pulse signal from the pulse signal source;
Enclosed in the same package as the semiconductor laser element, and the anode side is commonly connected to the cathode side of the semiconductor laser element, and receives a light from the semiconductor laser element and outputs a received light signal according to the received light amount. A light receiving element for monitoring,
A current control element that is connected between the pulse signal source and the semiconductor laser element and adjusts an amount of current supplied to the semiconductor laser element;
A drive signal corresponding to a potential level in a connection line between the pulse signal source and the current control element and a light reception signal level from the monitor light receiving element is supplied to a control terminal of the current control element to the semiconductor laser element. Feedback control means for performing feedback control so that the supply current of becomes a predetermined value;
A semiconductor laser driving apparatus comprising: a semiconductor element that blocks current flowing from the current control element to an input side of the feedback control means.

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